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ALUMINIUM SULPHATE                               43

and alkali, which are detected and determined as in aluminium sulphate
(vide infra). The value depends on the content of alumina, acetic acid
and sulphuric acid : the alumina and sulphuric acid are determined as
in aluminium sulphate and the acetic acid as in calcium acetate (q.v.).

The compositions of the basic acetates and sulpho-acetates of aluminium
vary considerably. The sulpho-acetate should preferably contain I part
of S03 per 2 parts of AlaOa.

The various aluminium acetates should be free more especially from
iron and zinc (sac also Aluminium Sulphate).

A18(S04)3 + i8HaO = 666-3

This exists in commerce in various forms : white, crystalline, nacreous
scales ; in lumps or cubical or prismatic cakes, which are hard, white and
opaque ; in spongy, anhydrous, white masses. In the first two forms it
is easily and completely soluble in water giving an acid solution; the
anhydrous form dissolves slowly and often leaves an insoluble residue of
basic sulphate.

The most, frequent impurities are small quantities of iron salts and free
sulphuric acid (to 'be tested for more especially when the product is to serve
for dyeing purposes), insoluble, substances (silica, sand) and rarely zinc,
copper, lead, chromium, titanium and arsenic.

Analysis consists essentially of determinations of the insoluble matter,
alumina, free, and total sulphuric acid and iron and is carried out as described
below. Tests for the other impurities mentioned above are made by the
ordinary met]iods.

The sample to 'be analysed is powdered rapidly and weighed in a closed

1.  Insoluble Matter.—-A solution of 20 grams in water is filtered
through a filter previously dried at 105° and weighed.   The insoluble
matter is thoroughly washed, dried at 105° and weighed.   The filtrate is
made up to 500 c.c. and used for the following determinations.

2.  Alumina........This is determined in 50 c.c. of the solution (2 grams

of material) by precipitating with ammonia and weighing as A18031; the
weight found is diminished by that of the Fea03 found as in 5 (below).

3.  Total Sulphuric Acid.—This is determined in 25 c.c. of the solution
(i gram of material) as barium sulphate,    i part of BaS04 — 0-34335
part of vS()a — 0-4206 part of HaSO«.   From the result is deducted any
free acid found (4).

From the relation between the alumina and the sulphuric acid is calcu-
lated the basicity of the product, from which it is deduced whether the
sulphate is normal or more or less basic. By basicity number is meant the

1 In order to avoid the action of organic substances, which may occur in the com-
mercial sulphate, Delage (Ann, de chim. analyt., 1911, p. 325) recommends the addition
of a few drops of bromine to the solution, which is then evaporated to dryness, heated
slightly until the residue is completely decolorised, then taken up in water and pre-
cipitated with ammonia.or iron and free* sulphuric